I resoldered mine when it came loose a few years back. I make sure now that the washer is in good condition and don't tighten it very much. No problem since and hoping it stays that way.
As for silver soldering, there are 3 golden rules to do that job:
1. The job MUST be clean.
2. The job MUST be clean.
3. I'm sure you get the idea!
To get the area hot enough for silver to melt, the lead/tin solder has burned to a mess and I don't think silver will flow properly. I think I'd completely remove the boss into which the plug screws from the sump, clean it (maybe by blasting) and go from there, maybe welding rather than silver
What a PITA it was when mine let go, I was 2,500 miles fro home and the car HAD to be serviced. I was in an empty warehouse alone and had to remove the sump to drain the oil, refit and refill. Why is it that these things always go wrong at the worst possible moment?
